In the fabrication or screening of full color luminescent faceplates for entertainment tubes and display tubes, blue, green and red phosphors are deposited consecutively on a glass faceplate to form a cyclic or triad pattern of phosphor dots or lines. In order to obtain high resolution, the pattern must be regular and uniform, with few missing or overlapping phosphor lines or dots. The phosphor materials that are applied must be finely divided and free of aggregated particles. Aggregates do not adhere to the glass faceplate, causing voids or missing dots or lines, or they may overlap an adjacent phosphor line or dot, causing a loss of color purity. These problems are exacerbated in color display tubes, where the average dot diameter is only 5-6 mils rather than the 10-12 mil line diameter used on entertainment tubes. The presence of holes or missing dots or lines require scrapping the resultant screened faceplate or finished tube. Thus the deaggregation of phosphor particles enhances packing density, reduces porosity, and provides resistance to cross contamination of the different adjacent phosphor colors.
It is conventional in the art to form an aqueous dispersion of the phosphor particles and various coating reagents, including: pigments to enhance the color contrast; latex or other binder to promote adhesion of the pigment to the phosphor; and particles of silica or metal hydroxide, such as zinc hydroxide, which impart a negative charge to the phosphor particles, reducing the tendency of the particles to aggregate.
After the phosphor has been coated, it is dried for storage and shipment. In the picture tube factory, the coated phosphor is slurried again, preferably in an aqueous polyvinyl alcohol (PVA) solution or an equivalent photopolymer. The resultant slurry is used in screening the desired pattern onto the faceplates. This PVA slurry is milled to reduce aggregation prior to printing.
However, milling, although effective to reduce phosphor particle size, is undesirable in several respects; it damages the phosphor crystals, reducing luminescent properties such as light efficiency; produces fines or small size particles because milling acts on all size particles equally; some of the coating is removed during milling; and it is a lengthy, up to 8-10 hours, process.
Treatment of phosphor dispersions with ultrasonics to reduce aggregation has previously been tried. However, up till now, this has not been successful. Use of a static containment system such as a bath has allowed aggregated phosphor particles to settle out of suspension and thus they do not get treated by the ultrasonic agitation.
Use of high speed mixing equipment to deaggregate phosphor particles has also been tried but is also unsatisfactory because it results in crystal damage and loss of coating materials.